The present invention relates to a method for fabricating a region for isolating electronic devices formed in a semiconductor substrate, and in particular to a method for providing an isolation region having a smooth upper surface.
In an integrated circuit where many devices reside on the same semiconductor substrate, regions in the substrate surround the devices to isolate them from each other. -n the well-known "isolation with oxide and polycrystalline silicon" technology, a trench is etched into a silicon substrate around an area. The inner surface of the trench is oxidized to form an insulating layer and the trench is filled with polycrystalline silicon (polysilicon). The exposed surface of the polysilicon near the top of the trench is covered with another insulating layer. The insulating layer above the trench isolates a later deposited polysilicon or other conductive layer from the polysilicon in the trench. A conductive layer above the trench is normally etched to provide signal paths over the trench for interconnecting a device fabricated in the isolated area with other devices of the integrated circuit.
The insulating layer above the aforementioned trench should have a smooth surface free of grooves. Otherwise when a conductive layer deposited on the insulating layer is etched to form signal paths over the trench, conductive material remaining in the grooves can cause unwanted shorts between signal paths.
U.S. Pat. No. 4,509,249, entitled "METHOD FOR FABRICATING ISOLATION REGION IN SEMICONDUCTOR DEVICE", issued Apr. 9, 1985 to Goto et al., discloses two methods for fabricating an isolation trench. In the first method, an insulating layer is formed above the trench by oxidizing the polysilicon in the trench. Goto et al teach that an insulating layer formed by oxidation has an irregular surface and therefore recommend another method for forming the insulating layer. In the recommended method, a thick insulating layer of silicon dioxide is deposited (rather than formed by oxidation) on the polysilicon. Goto et al point out that the second method yields a smoother upper surface on the insulating layer above the trench than when the insulating layer is formed by oxidation. However, when the silicon dioxide insulating layer is deposited on the polysilicon in the trench, it is also deposited everywhere else on the substrate and must be etched back. The process of forming the silicon dioxide insulating layer over the trench by deposition and etching is more complex and expensive than forming the insulation layer simply by oxidizing the polysilicon. In addition, the etchback process must be carefully controlled to avoid leaving a central depression in the insulating layer that can cause unwanted shorts in the integrated circuit.